Is there another way to make a coloured topography?
You could try something like this:

This will render in the viewport using the blender internal rendering engine, with GLSL shading (under the "shading" section of the tools menu ("N") of the 3D viewport), if you set the viewport to Textured render (ALT+z).

The "Orco" geometry input works such that the highest point on the mesh (including stray/hidden vertices) along the local z axis is red, and the lowest point is blue. It will not update in edit mode; tabbing in and out will change it.

If you want the gradient along the y or x axes instead, just change the output of the "seperate RGB" node (working here as a separate vector XYZ node) to G or R respectively.

You may need to modify your model such that the local z axis is aligned with the same direction you want your gradient.

Hope that helps!